This invention relates, in general, to the etching of silicon nitride, and more particularly to differential etching of silicon nitride employing hydrogen implants.
Typically, it has been extremely difficult to obtain a high quality differential etch of silicon nitride. Conventional masking and etching techniques generate uneven profiles having poor aspect ratios. In other words, although a differential etch may be obtained in the vertical direction, the etchant also etches horizontally under the masked regions of the silicon nitride. This causes difficulty in the fabrication of high performance electronic devices. Further, additional problems are encountered because there is generally low tolerance in stopping the etch and overetching is not uncommon.
Fabrication methods for self-aligned gate FETs have generally employed differential silicon nitride etching as disclosed above. Because of the problems associated with differential silicon nitride etching, some prior art self-aligned gate device fabrication methods employ the early formation of a metal layer which is etched to form gate metal. Further, dummy gates are commonly generated by requiring multiple dielectric-layers. This requires additional process steps.